1. Field
The present method relates generally to methods of determining the mechanical axis of a patient's limb using anatomical information and more particularly, to methods of providing a customized surgical device configured to guide a cutting tool along a plane perpendicular to the true, three-dimensional mechanical axis of a patient's limb.
2. Related Art
The mechanical axis of the human leg in the frontal plane is defined as a line drawn from the centre of the femoral head to the centre of the ankle joint. In a frontal plane, it normally passes just medial to the centre of the knee joint in the frontal plane. This line assumes sphericity in the femoral head and normal anatomy in the subtalar complex. In a sagittal plane, the normal mechanical axis runs from the centre of gravity, to the centre of the ankle joint. It therefore runs just behind the femoral head (because the femoral neck is generally anteverted about 15°) and just in front of the knee.
FIG. 1 illustrates the mechanical axis of a deficient lower limb and a normal lower limb. FIG. 1a illustrates a deficient lower limb, wherein a gross malalignment (12) between the femoral (14) and tibial (16) mechanical axes is visible. The mechanical axis of the limb (18) does not pass through the centre of the knee joint (20) as it should. FIG. 1b illustrates a normal lower limb, wherein the femoral (14) and tibial (16) mechanical axes are generally aligned and pass through the centre of the knee joint (20). The transverse axis (22) is aligned with the distal femoral contact points and is generally perpendicular to the femoral and tibial mechanical axes (14, 16). The mechanical axis is generally aligned with the femoral (14) and tibial (16) mechanical axes.
It has generally been convention to design standard knee prosthetics to accommodate a wide variety of people within a population. However, not all patients fall within this population subset, nor can all patients achieve the same performance with a universal or standard implant. In one instance, a patient may have a severe varus or valgus deformity, dwarf condyle(s), excessive femoral bowing, or bony tumour(s) present. In another instance, a patient may have abnormally small or large bone structure. In these circumstances, a custom implant may be desirable to a surgeon. However, current methods of manufacturing custom implants are generally expensive, require specialized input from a surgeon, and are time-consuming procedures.
Medical devices, in particular femoral cutting blocks, have been used to prepare the distal portions of a femur in total knee arthroplasties (TKA's). Such cutting blocks are typically mounted to the femur after at least a first resection has been made, said at least first resection being facilitated by an adjustable instrument referenced by and extending from an intrarmedulary or extramedulary rod. The instruments are made adjustable so that they may be used universally between patients. While there are many benefits to adjustable instruments, there are also many disadvantages. Some disadvantages include increased overhead, bulky kits and containers, unnecessary or redundant instruments, large numbers of trials and different sizes, increased OR time, increased sterilization time between surgeries, and increased financial risks to orthopaedic manufacturers which keep large numbers of assets on consignment.
Recent attempts have been made to streamline the surgical process and avoid the aforementioned disadvantages of standard resection instruments. Such methods have employed customized cutting blocks formed using anatomical information derived from a partial CT/MRI scan of a patient's knee joint. Corporations like OtisMed Corp. and ConforMIS, Inc. utilize such methods. However, to this end, it is believed that conventional customized cutting blocks and methods of distal femoral resection associated therewith do not ensure that at least one bony resection is made perpendicular to the true, three-dimensional mechanical axis of a patient. Instead, it is believed that the prior art approximates the mechanical axis by extrapolating a fixed number of degrees from the small anatomical axis portion visible in a partial scan of the knee joint.
For instance, if a partial knee CT scan or X-ray of a patient indicates a pre-operative anatomical axis at or around seven degrees, a conventional custom cutting block of the prior art will generally be formed to provide a distal femoral resection perpendicular to an axis displaced medially from said anatomical axis by a specified angle or number of degrees relative to the joint centre. The specified angle or number of degrees is believed to be chosen to reflect the mean deviation between anatomical and mechanical axes for a large population set. Said mean deviation has been generally accepted within the medical field as approximately 5-6 degrees. Therefore, using this methodology of the prior art, the mechanical axis in the example above would be approximated as being 1 degree from vertical, or about “seven degrees minus six degrees”. It is believed that practicing this method does not accurately place femoral resections perpendicular to the true mechanical axis, since the relationship between anatomical axis and mechanical axis may greatly differ between patients. Furthermore, if there are any unnoticed deformities of the distal tibia or proximal femur present, such assumptions could carry gross consequences. Lastly, since the true mechanical axis is defined in three dimensions, simply rotating a resection plane medially a predetermined number of degrees from the anatomical axis in a coronal plane without adjusting for anterior-posterior slope in a sagittal plane would not place said resection plane perpendicular to the true, three-dimensional mechanical axis.
While it is generally accepted that for a majority of people, the mechanical axis of a lower limb averages five to six degrees medially from the anatomical axis of the femur, and that the intersection of the femoral and tibial mechanical axes at the knee subtend an average of 1.3 degrees varus (±2 degrees) and intersect just medial to the centre of the joint, these observations cannot be relied upon as fact or used in any sort of algorithm in determining proper resection. In fact, it has been determined through research that such relationships between anatomical axis and mechanical axis of the femur may range from 2-11 degrees.
Thus, benefits may exist in ascertaining the true, three-dimensional mechanical axis of a limb prior to surgery, to determine the best placement of a surgical device.